Gas chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas or liquid are adsorbed or absorbed and then desorbed by a stationary phase material into a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into the chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various components of the sample—which differ based upon differences among partition coefficients of the elements—cause the sample to be separated into the respective elements. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents, and the specific concentrations thereof, which are present in the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
Typically, it is desired to pre-concentrate the analytes in the sample, and occasionally, remove moisture therefrom, prior to introducing the sample into the chromatographic column. Accordingly, as disclosed in U.S. Pat. Nos. 5,792,423 and 6,395,560 to Markelov, these systems will typically include some kind of “trap” for this purpose, which retains the analytes as they are carried through the trap, and then later releases these analytes, usually by heating, which are then swept into the chromatographic column.
Various types of traps have been suggested to perform this pre-concentration (and possible moisture removal) prior to introducing the sample into a chromatographic column. One type of trap, and a type that is particularly suited for removing moisture from the sample, is an adsorbent trap, which adsorbs the analytes as the sample is passed through it, which can then later be desorbed. Accordingly, numerous arrangements employing such traps have been employed for the purpose of pre-concentrating the analytes of a sample, which has typically been extracted by some kind of sampling device, such as, for example, a headspace sampler. Examples of such arrangements are disclosed in U.S. Pat. No. 5,932,482 to Markelov and U.S. Pat. No. 6,652,625 to Tipler.
However, to date, these systems have resulted in a number of disadvantages. First, in order to accomplish this multiple stage process of extracting and transferring a sample fluid to the trap, trapping it and untrapping it, and transferring it to the chromatographic column, these systems have employed complex assemblies of parts and/or valves situated in the flow path of the fluid containing the analytes to be measured. These extra devices and valves not only increase cost and space, but increase dead-volume areas and surface active sites. This results in sample dispersion, dilution, or loss, and causes excessive peak broadening on the chromatogram. Another disadvantage present in some of these systems is the uni-directional path of flow for both adsorption and desorption, inhibiting the ability to first trap heavier compounds and then more volatile compounds by using multiple adsorbents.
Accordingly, it has been proposed to use a system incorporating a trap where a carrier gas flows through the trap in one direction as it carries the sample fluid through the trap so that the adsorbent can adsorb the analytes to be measured, and a carrier gas flows through the trap in the opposite direction and carries the analytes out of the trap and to the chromatographic column as the analytes are thermally desorbed from the adsorbent, essentially as described herein. Additionally, it has been proposed to employ a “dry purge” step in between the aforementioned adsorption and desorption steps, where carrier gas flows through the trap in the same direction as it does during the initial adsorption (or “trap load”) step in order to purge from the trap any moisture that remained therein, essentially as described herein.
However, even when using these systems, some moisture invariably remains in the trap after the carrier gas has flowed therethrough during the adsorption and dry purge steps. The main reason for this is water condensation. For example, during headspace sampling, when the headspace vapor is transferred from a sample vial, it is saturated with water vapor at a high temperature (e.g., 85 degrees Celsius). During the adsorption (or trap load) step, this sample vapor (including the water vapor) enters the trap, which is maintained at a much lower temperature (e.g., 40 degrees Celsius). Because the saturation concentration of water in the vapor is directly proportional to its vapor pressure, there is immediate condensation of liquid water as the vapor pressure is reduced upon entry into the trap.
Similarly, the outlet tubing through which fluid is discharged from the trap after it has passed through the adsorbent is often at a lower temperature than the trap. Accordingly, condensation of water vapor will also take place in the outlet tubing. Therefore, even when a dry purge step is employed to try to sweep out residual water that remained in the trap after the adsorption (trap load) step, the dry purge step itself will result in some residual water.
Due to this condensation, one of the very objects that the trap seeks to achieve—namely, the elimination of unwanted moisture—is not fully accomplished. This is because the aforementioned outlet tubing also acts as an inlet for fluid during the desorption stage, serving as a supply line for carrier gas, which flows back through the trap to pick up the previously adsorbed analytes and sweep them back out of the trap and into the chromatographic column as the analytes are desorbed from the adsorbent. Because this carrier gas is flowing back over the areas where the water condensed, it sweeps this water back out of the tubing and the trap and into the chromatographic column along with the desorbed analytes.
What is desired, therefore, is a system for pre-concentrating analytes in a sample prior to introduction into a chromatographic column that is inexpensive to manufacture, does not take up a lot of space, and reduces the amount of dead volume areas and surface active sites. What is further desired is a system for pre-concentrating analytes in a sample prior to introduction into a chromatographic column that does not sweep into the chromatographic column water that has condensed during the adsorption and/or dry purge phases of the process. What is also desired is a system for pre-concentrating analytes in a sample prior to introduction into a chromatographic column that does not require substantial temperature control of multiple parts of the system.